create_dir: Create directories if they don't already exist
Description
Given the names of (potential) directories, create the ones that do not
already exist.
Usage
create_dir(...)
Value
Invisibly, a vector with a TRUE for each time a directory was
actually created and a FALSE otherwise. This vector is named with the
paths of the directories that were passed to the function.
Arguments
...
The names of the directories, specified via relative or absolute
paths. Duplicates are ignored.